In general, a method of removing sulfur dioxide (SO2) in the flue gas is divided into two processes, one of which is a wet process where SO2 gas is absorbed by an absorbing liquid and removed and the other one of which is a dry process where SO2 gas is adsorbed by an adsorbent and removed. When a large amount of flue gas containing high-concentration SO2 gas is treated, a wet process is broadly adopted. However, when a flue gas relatively low in concentration or small in amount is treated, in some cases, a dry process that is simple in structure and easy to maintain and manage can be adopted.
As a dry flue gas treatment process, a flue gas desulfurization process (catalytic desulfurization) where sulfur oxides such as SO2 gas and the like contained in a flue gas are oxidized with oxygen present at low temperatures to finally recover as sulfuric acid is known. When as such a catalyst that oxidizes SO2 gas and the like in the flue gas, a ceramic support such as alumina, silica, titania or zeolite is used, activity is deficient by itself; accordingly, as a catalyst species, metal or metal oxide has to be added. Furthermore, since the catalyst species suffers an attack from generated sulfuric acid to dissolve or denature, there is a disadvantage that the activity cannot be stably maintained over a long period of time. As the result thereof, as the catalyst, activated carbon that is excellent in acid resistance and thereby can maintain stable activity over a long period of time without undergoing deterioration has been most preferably used.
However, when commercially available activated carbon is used as it is as the catalyst, a problem is that catalyst activity in the catalytic desulfurization is low and generated sulfuric acid cannot be smoothly exhausted; accordingly, in order to obtain desired desulfurization effect, the catalyst has to be added much and regenerated periodically, resulting in poor economical efficiency.
In this connection, in, for example, Japanese Patent Application Laid-Open No. 2005-288380, a gas processing method capable of repeatedly processing odor components, air pollutants and the like in gas over a long period of time has been proposed. In the method, a gas to be treated is, after moisturizing so as to exceed 100% in relative humidity, brought into contact with an activated carbon-containing honeycomb or a chemical-supported activated carbon-containing honeycomb that supports a chemical such as iodine, bromine, acid, a platinum compound or the like so as to remarkably improve treatment efficiency.
According to the above-mentioned gas processing method, it is said that, when the relative humidity of a gas to be treated is controlled to a super-saturated state, that is, so as to exceed 100%, in a contact with an activated carbon-containing honeycomb, a thin water film is uniformly formed over a surface of the activated carbon-containing honeycomb, odor components and air pollutants are oxidized on a surface of the activated carbon-containing honeycomb to form compounds dissolvable in water, the water-soluble reaction products are gradually eluted through a water film from a surface of the activated carbon-containing honeycomb to detach from the activated carbon-containing honeycomb, thereby the activated carbon-containing honeycomb is self-regenerated to greatly lengthen a treatment life.
However, according to the gas processing method, separately, the relative humidity of a gas to be treated needs to be controlled by sprinkling or spraying water or an aqueous solution to the gas to be treated, or by bubbling the gas to be treated in an aqueous solution, followed such as by using a humidifier so as to exceed 100%; accordingly, a problem is that energy consumption necessary for gas treatment becomes larger.
Furthermore, in order to uniformly generate a thin water film on a surface of an activated carbon-containing honeycomb, the relative humidity is controlled so as to exceed 100%. However, a gas to be treated and the activated carbon-containing honeycomb are inhibited from directly coming into contact with each other to result in difficulty in exerting catalyst performance of the activated carbon; accordingly, another problem is that, an amount of the activated carbon-containing honeycomb necessary to obtain desired desulfurization effect becomes larger.
Furthermore, as described in the same literature that the activated carbon-containing honeycomb deteriorated in treatment capacity owing to long term usage can be repeatedly used by sprinkling water, a problem which remains is that the activated carbon-containing honeycomb itself necessitates a regeneration treatment owing to water sprinkling for every definite term. That is, a development of catalysts with higher activities is desired.
On the other hand, separately from a demand for a catalyst high in desulfurization activity, there is a following requirement.
That is, in a combustion flue gas exhausted from a boiler of a thermal power plant, in addition to generally contained SO2 gas, depending on a kind of fossil fuel (in particular, coal) to be combusted, in some cases, mercury is contained at high concentration. Mercury is a poisonous material that causes health hazards when exhausted in the environment; accordingly, the mercury has to be removed before releasing flue gas in air. Accordingly, recently, a restriction that makes removal of, in addition to SO2 gas, mercury compulsory has started.
In mercury in the flue gas, there is oxidized mercury (Hg2+) present in the form of divalent mercury compounds oxidized in a combustion furnace or by an oxidizing catalyst of NOx removal apparatus and elementary mercury) (Hg0) present in the form of simple (0-valent) metallic mercury. Among these, Hg2+ is almost removed by a flue gas desulfurization apparatus of wet system. However, Hg0 is low in solubility to an absorption liquid and thereby low in removal efficiency; accordingly, at the present time, almost all thereof is not removed and is diffused into the air.
In this connection, a method of more oxidizing Hg0 in a flue gas to Hg2+ by adding a halogen compound such as hydrogen chloride, calcium bromide or the like to a flue gas or coal that is a fuel or by making use of an oxidizing catalyst of a NOx removal apparatus has been proposed (Japanese Patent Application Laid-Open No. 2004-66229). However, there is a problem with catalyst lifetime and moreover since the diffusion of Hg0 in the flue gas becomes rate-determining, it is difficult to achieve a high oxidizing efficiency. That is, it is difficult to stably oxidize Hg0 to Hg2+ at high efficiency over a long period of time.
Furthermore, also a method of adding a Hg fixing agent such as a chelating agent, a potassium iodide (KI) solution or the like to an absorption liquid of a wet flue gas desulfurization apparatus, or adding an oxidizing agent such as hypochlorous acid, hydrogen peroxide or the like has been proposed (Japanese Patent Application Laid-Open No. H10-216476). However, the Hg fixing agent or oxidizing agent is decomposed via a reaction with other metal, consumed in oxidation of SO2 gas in the flue gas, or volatilized and diffused from a stack; accordingly, a problem is that an added amount of the adding agent increases. When a chelating agent is added, another problem is that the chelating agent is decomposed to generate hydrogen sulfide (H2S) to diffuse bad odor.
In a method where various kinds of additives are added to an absorption liquid, it is known that, when a state of the absorption liquid is varied depending on a variation of an electric generation load or a variation of a flue gas composition, Hg0 absorbed once in the absorption liquid is re-released or Hg2+ in the absorption liquid is reduced to Hg0 and released; accordingly, also a technology that does not re-release Hg0 is under development (Japanese Patent Application Laid-Open No. 2004-313833). Furthermore, in a method where an oxidizing agent such as hypochlorous acid, hydrogen peroxide, chromic acid, or chlorine is used, a reaction between an oxidizing agent and SO2 gas in the flue gas cannot be avoided to result in a large loss of the oxidizing agent; accordingly, it has been proposed to spray the oxidizing agent on a gas downstream side of a flue gas desulfurization apparatus (Japanese Patent Application Laid-Open No. 2001-162135).
On the other hand, as a method of removing Hg0 not by absorbing in an absorption liquid of a wet flue gas desulfurization apparatus but by a separate method, a method where a powder of activated carbon is added and dispersed in a flue gas in a gas region where a temperature is about 100 to 150° C. and the Hg0 is adsorbed by activated carbon powder to remove has been known (Japanese Patent Application Laid-Open No. H9-308817). Furthermore, it has been known for long that activated carbon supporting bromide or the like is effective in removing mercury (Japanese Patent Application Laid-Open Nos. S49-53590 and S43-53591). However, mercury adsorption capacity of the activated carbon is generally low; accordingly, from the viewpoint of uniform contact, unless an added amount to the flue gas is increased, an advantage cannot be obtained. As the result thereof, the activated carbon much added into the flue gas has to be collected together with fry ash on a downstream side, and for this, a large electrostatic precipitator has to be installed, and an apparatus for processing the activated carbon collected in a state mixed with fry ash is necessary. The methods are applied on an upstream side of wet flue gas desulfurization apparatus or used in combination with a dry or semi-dry flue gas desulfurization apparatus to remove mercury contained in the flue gas at certain extent high concentrations. That is, the methods do not remove low concentration mercury such as contained in an exit gas of the wet flue gas desulfurization apparatus.
On the other hand, a method where activated carbon supporting iodine or the like is brought into contact with an exit gas of a wet flue gas desulfurization apparatus, in more detail, an exit gas of a wet electrostatic precipitator disposed on a downstream side of a wet flue gas desulfurization apparatus, to remove mercury in the flue gas has been proposed (Japanese Patent Application Laid-Open No. H10-216476). However, according to the method, a wet electrostatic precipitator is disposed on an upstream side of the mercury removing apparatus; accordingly, an exit gas of the wet flue gas desulfurization apparatus does not contain mist in the flue gas. Furthermore, a gas re-heater is used to elevate a temperature to 77° C. or more. That is, by elevating a temperature to lower the relative humidity, in essence, after a condition close to that of an upstream side of the wet flue gas desulfurization apparatus is established, a process is conducted with iodine-supported activated carbon.
As mentioned above, a problem of a conventional method where mercury in the flue gas is absorbed by an absorption liquid of wet flue gas desulfurization apparatus and removed is that it is difficult to stably maintain high mercury removal efficiency over a long period of time. Furthermore, an oxidizing agent for oxidizing mercury is consumed to oxidize SO2 gas or a chelating agent for collecting mercury reacts with other metal to cause big loss; accordingly, another problem is that an added oxidizing agent or chelating agent is not effectively used, mercury is insufficiently oxidized, and Hg0 is re-released from the absorption liquid.
On the other hand, in a method where activated carbon powder is dispersed in the flue gas to adsorb and remove mercury, as mentioned above, an addition amount of the activated carbon becomes larger because of small in the mercury adsorption capacity of the activated carbon; accordingly, a problem is that the cost is disadvantageous when the cost of post-processing is included. Furthermore, when a concentration of water vapor or SO2 gas in the flue gas is high, the mercury adsorption capacity of the activated carbon is remarkably lowered, and, even when activated carbon supporting a halogen compound such as a bromine compound or the like is used, sufficient adsorption capacity cannot be obtained; accordingly, in the case of using in combination with a wet flue gas desulfurization apparatus, when processed on an upstream side thereof, SO2 gas affects greatly, and when processed on a downstream side, water vapor greatly affects, that is, when a treatment is conducted on either side thereof, there is a dilemma that a large decrease in the adsorption capacity of the activated carbon cannot be avoided. Accordingly, in many cases, adsorption treatment with activated carbon is combined with the dry flue gas desulfurization. That is, it is not usually assumed to combine with wet flue gas desulfurization high in desulfuration efficiency, in particular, to use on a downstream side of the wet flue gas desulfurization apparatus.